This invention relates to transcutaneous energy transfer (TET) devices and, more particularly, to an improved primary coil for such device which reduces sensitivity to conductive objects in proximity of the TET.
Many medical devices are now designed to be implantable, including pacemakers, defibrillators, circulatory assist devices, cardiac replacement devices such as artificial hearts, cochlea implants, neuromuscular simulators, biosensors, and the like. Since almost all of the active devices (i.e., those that perform work) and many of the passive devices (i.e., those that do not perform work) require a source of power, inductively coupled transcutaneous energy transfer (TET) devices and information transmission systems for such devices are coming into increasing use.
These systems generally include an external primary coil and an implanted secondary coil, separated by an intervening layer of tissue. This design generally results in a loosely-coupled transformer with no magnetic shielding. Therefore, transformer parameters, such as mutual and self-inductance values, and the effective series resistance of each coil, can be altered by the presence of conductive objects, for example a metal plate, in the vicinity of the primary coil. Such parameter changes can result in undesired, and in some cases potentially catastrophic, variations in power delivered to the implanted device. Further, an unshielded primary coil generates a magnetic field which is directed in substantially equal parts toward the secondary coil, where it performs useful work, and away from the secondary coil where the magnetic field energy is substantially wasted. If a higher percentage of the magnetic field from the primary coil could be directed to the implanted secondary coil, the energy required to drive the TET device could be reduced. This could result in the device being driveable from a lower energy, and thus a smaller, lighter and less expensive source, or less drainage on could result in an existing source, facilitating longer battery life between replacement or recharging.
A need therefore exists for an improved primary coil construction for a TET device which both reduces sensitivity of the device to conducting objects in the vicinity of the coils and, preferably, which increases the percentage of magnetic field generated by the primary coil which reaches the secondary coil. Such a device and method would significantly enhance the energy transfer efficiency of the TET device.
In accordance with the above, this invention provides a transcutaneous energy transfer device having an external primary coil to which energy to be transferred is applied and an implanted secondary coil inductively coupled to the primary coil and connected to apply energy to a subcutaneous utilization device, the invention being characterized by the inclusion of a magnetic shield covering the primary winding. The shape of the shield is generally substantially the same as that of the primary coil, but the size of the shield should be greater than that of the primary coil. More particularly, to fully reflect magnetic field toward the secondary coil, the shield should overlap the primary coil on all sides by at least the thickness (t) of the shield. Where the primary coil has a generally circular shape with a diameter d, the shield has a generally circular shape with a diameter D, where D greater than d and preferably Dxe2x89xa7d+2t. The thickness of the shield for a circular shield is preferably much greater than D/xcexc where xcexc is the magnetic permeability relative to free space of the shield material, or more generally, t greater than  greater than Xxcexc, where X is a major dimension of the shield.
The shield normally has a plurality of ventilation perforations formed therein which perforations are preferably formed parallel to the magnetic field direction so that the path taken through the material of the shield is as short as possible. For embodiments where the primary coil is circular, the perforations are a plurality of radial slots, which slots are slightly wedge-shaped for an illustrative embodiment. To assure adequate ventilation, the perforations should make up between approximately 25% and 75% of the shield area. Since the perforations reduce xcexc of the shield material, for the shield thickness to continue to satisfy t greater than  greater than Xxcexc, t needs to increase proportionally (i.e., if the shield is 50% perforated, shield thickness tp=2t). Perforation size should also be small compared the smallest coil in the TET device.
The shield should also be flexible so as to be able to conform to the contours of a patient""s body. To achieve this flexibility, for one embodiment of the invention the shield is formed of a low loss magnetic material in a flexible polymer matrix, the shield being formed of a ferrite powder in a silicon rubber for an illustrative embodiment. For another embodiment, the shield is formed of a plurality of segments of a very high permeability material connected by a porous, flexible material. To the extent there are spacings between adjacent segments in a direction substantially perpendicular to the primary coil magnetic field in order to enhance flexibility, such spacings are much smaller than spacings in a direction parallel to the magnetic field.
The shield is also dimensioned and formed of a material which reflects most of the magnetic field directed away from the secondary coil back toward the secondary coil. This significantly enhances the efficiency of energy transfer across the skin boundary by the TET device.
In one embodiment the segments include a plurality of segments arranged in one or more concentric rings, each said concentric ring including segments of substantially the same size. In another embodiment, the plurality of segments are constructed and arranged so as to form a gap between radially opposing segments in said ring. In this embodiment, the segments further include a center disk shaped to fit within said gap.
In anther aspect of the invention, the shield and said primary coil are mounted together to form a primary coil assembly. A substantially impervious coating is applied to the assembly to make it substantially waterproof and easy to clean. In one particular embodiment, the primary coil assembly is vinyl dip coated.
In another aspect of the invention, the primary coil is operationally decoupled from a drive circuit prior to physical disconnection of electrical contacts through which current is transferred from the drive circuitry to said primary coil. The physical connection of electrical contacts through which current is transferred from the drive circuitry to said primary coil occurs prior to operationally coupled the primary winding to the drive circuit. In one implementation, the primary coil is electrically coupled to the drive circuit via an electrical connector. The electrical connector includes power transfer contacts and anti-arcing contacts. The anti-arcing contacts electrically mate after and break before said power transfer contacts, and are electrically connected to control circuitry operationally interposed between the drive circuit from the primary winding. In one embodiment, the control circuitry is located in said drive circuitry while in another embodiment, it is located in the connector.